Determination of Melt Flow Rate of Nitrile Butadiene Rubber (NBR): Standards, Methods, and Applicati

Melt flow rate (MFR) determination is a key test for evaluating the processability and batch-to-batch consistency of nitrile butadiene rubber (NBR) and its compounds. This technical article explains the principles, applicable standards, sampling requirements, test procedures, and typical applications of MFR and MVR measurement for NBR materials in quality control and research.

Technical Overview

Nitrile butadiene rubber (NBR) is a synthetic elastomer copolymerized from acrylonitrile and butadiene, widely used for seals, hoses, gaskets, fuel-resistant parts, and oilfield components. Because the flow behavior of unvulcanized NBR directly determines mixing, extrusion, calendering, and molding performance, an accurate and reproducible measure of melt fluidity is essential for both incoming material inspection and compound development.

Melt flow rate (MFR), also known as melt flow index (MFI), quantifies the mass of polymer extruded through a standard die under a specified temperature and load over a fixed time, expressed in grams per 10 minutes. The companion parameter, melt volume flow rate (MVR), is expressed in cubic centimeters per 10 minutes and is obtained by measuring piston displacement rather than extrudate mass. For NBR and NBR-based thermoplastic compounds, these measurements are typically performed according to ISO 1133-1 and ASTM D1238 using an extrusion plastometer. Results provide a low-shear comparison of flowability that correlates with molecular weight, plasticizer content, filler loading, and overall processing ease.

Testing Samples

Raw Unvulcanized NBR

Neat NBR gum in bale, crumb, or powder form can be evaluated directly. The material is cut or diced into small pieces (approximately 3 to 5 mm) to ensure uniform charging into the barrel. Because NBR can undergo thermal crosslinking or degradation at elevated temperatures, test conditions must be selected to keep the exposure time short.

NBR Compounds and Masterbatches

Filled, plasticized, or otherwise compounded NBR stocks may be tested to verify mixing uniformity and to compare processability between production batches. Samples should be free of trapped air, moisture, and volatiles, and should be conditioned in a controlled environment before testing.

NBR/PVC Blends and NBR-Based Thermoplastic Elastomers

NBR/PVC blends and thermoplastic elastomers based on nitrile rubber are processed like thermoplastics, making MFR and MVR particularly relevant for injection molding, extrusion, and blow molding applications. Sample geometry may be pellets, granules, or diced strips.

Hydrogenated NBR (HNBR) and Specialty Grades

HNBR and other modified nitrile grades can also be characterized, provided the test temperature is low enough to avoid premature crosslinking during the preheat and measurement period.

Testing Items

Standard test items reported for nitrile rubber melt flow characterization include:

• Melt flow rate (MFR), expressed in g/10 min at a stated temperature and load
• Melt volume flow rate (MVR), expressed in cm³/10 min at a stated temperature and load
• Melt density, derived from the ratio of MFR to MVR
• Flow rate ratio (FRR), the ratio of flow rates measured at two different loads, indicating shear sensitivity
• Optional thermal stability assessment to confirm that no crosslinking occurs under test conditions

Testing Methods

Apparatus

The test is performed with an extrusion plastometer consisting of a vertically mounted, temperature-controlled steel barrel with an internal diameter of approximately 9.55 mm, a weighted piston, and a calibrated die with a bore of 2.095 mm diameter and 8.00 mm length. Barrel temperature is maintained within ±0.5 °C of the set point, and the piston and die are cleaned and inspected before each run.

Sample Preparation

A charge of 3 to 8 g is used per determination. The sample is cut into uniform pieces, dried if necessary, and pre-conditioned at ambient conditions in a desiccator or controlled laboratory atmosphere. Contamination, moisture, and prior thermal history must be minimized, as they strongly influence flow results.

Procedure (Method A — Mass Measurement)

• Stabilize the apparatus at the selected temperature, typically in the range of 150 to 190 °C for NBR and NBR/PVC compounds, as agreed between the interested parties.
• Charge the sample into the barrel, compact it, insert the piston, and allow the preheat period specified by the standard.
• Apply the selected load, commonly 5.00 kg or 10.0 kg for nitrile materials.
• After the designated start time, cut off and discard the initial extrudate, then collect successive timed cuttings.
• Weigh each cutting to the nearest 0.001 g or better and calculate MFR using the formula MFR = 600 × m / t, where m is the average mass in grams and t is the time interval in seconds.

Procedure (Method B — Displacement Measurement)

With an automated instrument, an optical or magnetic transducer tracks piston travel. The volume extruded per unit time is converted to MVR in cm³/10 min, and melt density is calculated by dividing MFR by MVR. Method B eliminates cutting and weighing errors and is preferred for materials with very low flow rates.

Quality Control of Results

At least three determinations are recommended. Extrudate must be free of bubbles, and cut-off masses should agree within the repeatability limits of the standard. If the extrudate shows signs of crosslinking, foaming, or degradation, the test temperature and residence time must be reduced and the run repeated.

Applications

• Incoming inspection of raw NBR to verify supplier consistency against specifications
• Batch-to-batch processability control in rubber mixing and compounding operations
• Formulation development, including evaluation of plasticizer level, filler loading, and molecular weight effects
• Process window definition for extrusion, injection molding, calendering, and transfer molding of NBR/PVC thermoplastic blends
• Comparative benchmarking of competitive nitrile grades and failure analysis of flow-related processing defects
• Support for certification, datasheet generation, and regulatory or customer documentation

FAQ

What test conditions are normally used for NBR?

Because NBR grades vary widely in acrylonitrile content and compounding, conditions are agreed between the customer and laboratory. Commonly used combinations fall between 150 and 190 °C with loads of 5.00 kg or 10.0 kg, chosen so that measurable flow occurs without thermal crosslinking.

What is the difference between MFR and MVR?

MFR is the mass extruded in 10 minutes (g/10 min) obtained by weighing cut-offs, while MVR is the volume extruded in 10 minutes (cm³/10 min) obtained from piston displacement. The two are related through melt density.

How does MFR relate to Mooney viscosity for nitrile rubber?

Mooney viscosity (ASTM D1646 / ISO 289) is the traditional processability index for rubbers and measures torque at low shear, whereas MFR measures capillary flow at a defined shear stress. Both reflect molecular weight and structure; a higher MFR generally corresponds to a lower Mooney viscosity and easier flow.

How much sample is required and what is the typical turnaround?

Approximately 20 to 50 g of material is sufficient for replicate determinations, and results are typically issued within three to five working days after sample receipt.

What factors most affect result accuracy?

Temperature stability, die and piston cleanliness, moisture and volatiles in the sample, thermal degradation or crosslinking during preheat, and operator cutting technique are the dominant sources of variability, all of which are controlled through standardized procedure and instrument calibration.